1. Field of the Invention
The present invention relates to a high efficiency process for the obtaining of neutral compounds from black-liquor soaps or crude tall oil (CTO) or any fraction obtained from distillation of crude tall oil that can be used as raw material for the elaboration of pharmaceutical or alimentary products.
2. Description of the Related Art
Black-liquor soaps are by-products of the Kraft pulping process of pine and other woods. Typically, during the Kraft process, wood chips are digested or cooked for two hours at 170.degree. C. in aqueous liquor containing sodium hydroxide and sodium sulfide. The digestion delignifies the wood chips and gives rise to the production of cellulose pulp, sodium rosin soaps, sodium fatty soaps, lignin degradation products and a series of other chemicals present in the wood. At this conditions, the cellulose is stable and remains in suspension in the black liquor. At the end of the pulping process, the cellulose pulp is separated from the black liquor and washed. The washed pulp can be used as such or be further processed.
The cooking liquor, also known as black liquor must be recovered for both economics and environmental reasons. To this end, the black liquor is typically concentrated by evaporation to a solid content from 23 to 32% in weight, which causes the separation of fatty acid soaps and rosin soaps and also other hydrophobic compounds solubilized in the soaps, that float at the top of the vessel where they are removed or skimmed off. Hence they are called "skimming", a term that will be used as synonymous for black liquor soap or black liquor soap solution in the present invention because the skimming is a concentrated aqueous solution that also has a small amount of filtrable solids.
The skimming contains ordinarily between 30 and 50% of water. The skimming solids are a complex mixture of sodium fatty soaps and sodium rosin soaps, and a series of hydrophobic compounds, such as esters, sterols, stanols, fatty alcohols, waxes, terpenes and small quantities of inorganic salts, such as sodium sulfate and residual black liquor. These hydrophobic compounds of the skimming or black liquor soap solids are known as the unsaponifiable fraction or unsaps. Sometimes these unsaps constitute up to 30% and more of the skimming solids.
The skimming may be used as fuel oil; its calorific value is lower than the half of the fuel oil calorific value. It may be upgraded by transforming into tall oil. This is done by adding sulfuric acid and recovering the oil phase from the aqueous phase. This oil is known as crude tall oil (CTO). Then CTO is exposed to a series of vacuum distillations which produce a light fraction called HEADS, rich in unsaponifiable and fatty acids; fatty acids, TOFA or Tall Oil Fatty Acids, (these are the most valuable product obtainable from CTO); rosin acids or TORA (Tall Oil Rosin Acids); DTO or Distilled Tall Oil, which has many industrial uses and PITCH that is the bottom of the distillation, which is used as fuel or as an ingredient for the preparation of asphalt emulsions. Tall oil distillation processes are widely known in the state of art.
The presence of unsaps in the skimming notably reduces the quality of CTO and its by-products (TORA and TOFA), and it has a repercussion on the final purity of tall oil products which it can normally be detected by a CTO low acid number. A CTO with a high unsap content cannot be economically refined by means of vacuum distillation due to the formation of high proportion (up to 50%) of a low value pitch. Unsap components with a functional alcohol group, such as sterols, stanols, fatty alcohols and others can be combined with fatty acids during acidification of soaps to form esters and thus during CTO washing, drying and storage processes. Most esters have low volatility, which leads to the formation of a high pitch proportion during distillation, which produces a loss of the valuable fatty acids in the TOFA fraction. Unesterified unsap normally have a high volatility and are distilled off with fatty acids, reducing the acid number of TOFA and consequently lowering its value. Therefore, in order to increase the usefulness and value of the CTO and its by-products, it is necessary to refine the skimming to remove or separate the unsaps.
By this means, a wide variety of refining techniques has been developed; their primary objective is to obtain refined soaps to the detriment of the efficient recovery of both quality and quantity of neutral compounds. However, new uses and applications of several components of neutral compounds have lately appeared, notably sterols, stanols, and certain types of fatty alcohol such as docosanol and tetracosanol that can be found in important proportions in the unsaps (Table I). Sterols either free or esterified, have been revealed as important antitumor-like agents and their utilization for that purpose is described in the U.S. Pat. No. 5,270,041. The reduced form of stanols has an important application in the formulation of diets for the reduction of cholesterol plasmatic levels. The use of free stanols for that purpose appears in the U.S. Pat. No. 5,244,887, and the utilization of stanol esters for the same purpose appears in the U.S. Pat. No. 5,502,887.
TABLE 1 Unsaps average composition Compound Weight % 3,5-sitostadiene-3-ona 0.6 4-stigmasten-3-ona 0.5 .alpha.-sitosterol 0.6 .beta.-sitostanol 7.5 .beta.-sitosterol 21.0 Campestanol 0.6 Campesterol 2.1 Cycloartenol 0.5 Docosanol 4.4 Eicosanol 3.6 Ergosterol 0.2 Escualene 1.6 Fatty alcohol esters 6.1 Sterol esters 13.7 Hexacosanol 0.2 Methylencycloartenol 0.4 Pimaral 0.7 Pimarol 2.1 Stigmasta-3-ona 0.3 Tetracosanol 2.5 Others 30.8
There is an increasing interest about fatty alcohols, especially docosanol and tetracosanol, due to their notable pharmacological properties as both anti-inflammatory and antiviral agents. The use of these types of alcohol with pharmacological purposes have been disclosed in many U.S. patents (U.S. Pat. Nos. 4,874,794; 3,031,376; 5,534,554: 5,071,879; and 5,166,219). Therefore, it is supposed that in a near future, the unsaps are going to be considered as primary interest products instead of by-products of black-liquor soap or CTO (crude tall oil) refining. Nevertheless, due to the emphasis of refining methods in soap quality, these methods already known have many disadvantages for the efficient recovery of high-quality unsaponifiables.
Before 1978, a series of black-liquor soap refining methods had been developed, mostly based on the extraction of unsaps by organic solvents. Only U.S. Pat. No. 2,866,739 discloses a different method that consists of steam entraining of sterols and fatty alcohols from the pitch of tall oil distillation at temperatures between 190 and 280.degree. C., followed by vacuum rectification. This method has many defects, such as a thermal and oxidative deterioration of valuable compounds due to the utilization of steam at high temperature, but, above all, this method is incapable of separating the important amount of sterol esters that remain the distillate.
In regard to extracting methods with solvents, it is getting technically harder to fulfill to the increasing environmental restrictions for solvent contents. Therefore, maintaining a process inside the allowed limits of emissions and solvent contents considerably raises the cost of these unsap extracting methods.
As an alternative to unsap extracting methods with liquid solvents, the U.S. Pat. No. 4,422,966 discloses an extracting method of tall oil unsaps through supercritical hydrocarbons, principally ethylene. This method also has difficulties that are inherent in the utilization of solvents and high pressure, which further raises the cost of maintaining ethylene emission levels in the atmosphere within the limits of environmental laws. U.S. Pat. No. 4,076,700 discloses a refining method of skimmings and tall oil soaps. This method consists of feeding these skimmings or tall oil soaps to a thin film evaporator at temperatures between 230 and 310.degree. C. and pressures higher than 1 mmHg. As a first step, water and an unidentified light fraction of unsaps (possibly sterols and free fatty alcohols) are distilled off. Then, dehydrated and partially refined soaps are exposed to an exhaustive saponification that tends to hydrolyze sterol esters and fatty alcohols. The reason for the exhaustive saponification of raw materials is the incapacity of the disclosed method to separate esterified compounds through distillation. However, through the procedure of the present invention described below, it is possible to separate esterified compounds through distillation without saponificating the mixture.
Next, the resulting mixture is acidified and distilled again in a thin film column and fatty acids and isomerized rosin acids are obtained. In a variant of this method, skimmings and tall oil soaps are first exhaustively saponified to hydrolyze sterol esters and then they are exposed to the described process. One of the principal disadvantages of the revealed procedure is its incapability to separate sterol esters through distillation; for this reason, it appeals to the exhaustive saponification at high temperature and pressure. The distillation of both water and unsaps has many problems. In the first place, it represents an enormous load to the vacuum system, which seriously limits its capability of reaching high vacuums. In addition, simultaneous distillation of unsaps and water fractions causes the solidification of the former in the condenser which largely impedes its later recovery. When the saponification occurs in alcoholic solutions, the formation of emulsions that are hard to separate is produced in the condenser and it is necessary to perform later purification stages to obtain such unsaps. In addition, a high water proportion in the distillate impedes a raise of temperature over soap fusion temperature and as a cause of this, soaps adhere to the walls of the column. To relieve this problem, the column has a scraper with a tolerance level of practically 0, which rotates at about 1000 rpm with all the mechanical problems and the excessive energy consumption that this means.
U.S. Pat. No. 4,151,160 discloses a separating method of tall oil unsaps. This method consists of the formation of metallic cation soaps such as zinc and lead, followed by the conventional vacuum distillation at temperatures around 250.degree. C. and pressures over 0.5 mmHg. This method does not allow for separating sterol esters, and the presence of resulting water of the simultaneous neutralization and the distillation of water and unsaps causes the same problems in the vacuum system operation and in the products as described above. The unsap separating methods described in the present invention, correct all the problems that are inherent to the processes known in the state of the art and they provide a high efficient method for obtaining these unsaps.